Master cylinders known to the state of the art have a primary piston and a secondary piston generally made of aluminum, both of which are installed in series in an axial bore hole of a brake master-cylinder body, generally made of aluminum and machined. Such a master cylinder is described in French Patent No. 2,827,244. A push rod is used to actuate the displacement of the primary piston. The primary piston serves to pressurize a primary pressure chamber and a secondary piston serves to pressurize a secondary pressure chamber. Primary and secondary springs tend to push the pistons in the direction opposite displacement, thereby ensuring the increase in pressure.
The bore hole of the master cylinder is supplied with brake fluid from two supply access holes that are connected to a brake fluid reservoir. The supply access holes are used to supply the primary and secondary pressure chambers. These holes emerge in annular chambers, annular seals known as “cups” being provided on either side of the annular chambers.
The supply of brake fluid to the pressure chambers occurs when the pistons are at rest. The pistons are then in the position shown in FIG. 1. Supply occurs by passages provided in the piston walls, which enable the supply access holes and the annular chambers to communicate with the interior of the primary and secondary pistons emerging respectively in the primary and secondary pressure chambers. When the pistons are moved axially forward (direction of arrow D in FIG. 1), the piston passages cross the seals, isolating the supply chambers and enabling brake pressure to be established in the primary and secondary pressure chambers.
The master cylinder assembly is capable of being installed on a brake-assist servomotor. When the pistons are displaced along the direction of arrow D by a push rod that exercises a selective force on the primary piston, cup 4 isolates the primary pressure chamber of the primary supply access hole and cup 6 isolates the secondary pressure chamber of the supply access hole. When the force on the push rod is released, the volume of brake fluid accumulated in the brakes and springs of the master cylinder pushes the pistons into rest position. At times, when the push rod is rapidly released, the brake fluid contained in the pressure chambers of the master cylinder can drop below atmospheric pressure due to the action of the springs, which push the pistons more rapidly than the ability of brake fluid to flow through the master cylinder. When the pistons reach rest position, communication between the reservoir at atmospheric pressure and the chambers of the master cylinder is directly established and a sudden surge of brake fluid occurs, which generates noise in the master cylinder, known as a “fluid hammer.” To improve the performance of master cylinders, it is necessary to provide aluminum master-cylinder pistons with specific shapes, which shapes can result in significant additional costs due to the complexity of their production.